Hitherto, as for electrophotographic light-sensitive materials, inorganic photoconductive materials such as amorphous selenium, selenium alloys, cadmium sulfide, zinc oxide, etc., and organic photoconductive materials such as polyvinylcarbazole and polyvinylcarbazole derivatives are widely known.
Organic photoconductive materials have the advantages that they are superior in the points of transparency, film-forming property, flexibility, producibility, etc., to inorganic photoconductive materials.
Thus, recently electrophotographic light-sensitive materials using organic photoconductive materials have been variously investigated with keen interest. As these organic photoconductive materials, there are bisazo series pigments, trisazo series pigments, phthalocyanine series pigments, cyanines, pyryliums, etc., in addition to polyvinylcarbazole and the derivatives thereof described above.
However, these organic photoconductive materials are yet inferior to inorganic photoconductors in the points of sensitivity and durability.
Recently, it is reported that certain kinds of squarylium compounds show excellent photoconductive characteristics and can be used for electrophotographic light-sensitive materials as described in JP-A-60-136542, JP-60-142946, JP-60-142947, JP-61-10540, JP-62-450, etc. The term "JP-A" as used herein means an "unexamined published Japanese Patent application".
A squarylium compound is generally synthesized using dichlorobutenedione as a raw material.
In this case, it is known that 3,4-dichloro-3-cyclo-butene-1,2-dione (squarylic acid chloride) can form corresponding 3-aryl-4-chloro-3-cyclobutene-1,2-dione by the reaction with an aromatic compound in the presence of Lewis acid as shown by the following reactions (1) and (2) as described in B. R. Green et al, Synthesis, 46(1974) and L. A. Wennking et al, J. Org. Chem., 42(7), 1126(1977): ##STR3##
These reactions have a problem in selectivity. In particular, the reaction (2) has a problem of by-producing a 1,2-addition product (yield 3%) shown by formula (3) ##STR4##
Thus, the inventors have investigated for obtaining novel squarylium compounds having excellent photoconductive characteristics, in particular, high sensitivity, as well as novel cyclobutenedione derivatives which are used as intermediates for producing the squarylium compounds. As the result thereof, it has been discovered that the novel squarylium compounds described hereinafter are useful as a charge generating agent for electrophotographic light-sensitive materials and that novel cyclobutenedione derivatives are obtained by using specific tertiary aromatic amines, and the inventors have accomplished the present invention based on the discovery.